1. Field of the Invention
An object of the invention is a mobile telephone comprising an improved battery charger circuit. The field of the invention is that of battery-operated autonomous devices and especially that of autonomous devices that require a high degree of autonomy. This is the case in the mobile telephone. The purpose of the invention is to reduce the number of components and the cost of the battery charger circuit.
2. Description of the Prior Art
In the prior art, there is a known battery charging circuit whose input is a diode followed by a bipolar transistor or field-effect transistor. The diode is connected to the emitter of the bipolar transistor. Between the emitter and the base of the transistor, a resistor is connected and between the base and the ground, a transistor, which is for example a CMOS transistor, is connected. The collector of the bipolar transistor is connected to the battery. To charge the battery, the diode is connected to a power source, for example a current generator.
This circuit has two drawbacks. A first drawback is the presence of the diode which is very bulky and therefore hampers integration. Its presence is made necessary in cases when the input of the charging circuit might be shorted to ground. Indeed, the voltage which would then exist at the terminals of the emitter or collector of the bipolar transistor would be equal to the battery voltage which can be higher than 5 volts. In this case, the bipolar transistor would be destroyed and there would no longer be any other means of recharging the autonomous device.
The second problem is a problem of charge loss in the battery. Indeed, real transistors are not perfect and there is a leakage current between the collector and the base. This leakage current, while it is low, may be compared for example to the consumption of a mobile telephone in standby mode. The existence of this leakage current therefore reduces the autonomy of the portable telephone in the case of a short-circuit at input of the charger.
The invention resolves these problems firstly on the basis of the observation that, with present-day technologies, most batteries have a voltage of less than 5 volts. It is therefore no longer necessary to use a protection diode to prevent reverse voltages and overvoltages. Furthermore, the second problem is resolved by the series-connection of a switch with the resistor. This switch is parallel-controlled with the CMOS transistor. Thus, during the charging of the battery, the CMOS transistor will be on and the interrupter closed. When the battery is no longer being charged, the switch is open and the CMOS transistor is off. An alternative solution consists in placing a high-value resistor, of about 100 megohms, ensuring a low reverse current. Thus, according to the invention, between the emitter and the base of the bipolar transistor, there is infinite resistance when the battery is no longer charged. The leakage current therefore becomes zero. Furthermore, the resistor is sized so that the time constant of the charging circuit enables the battery to be charged in pulsed mode. The invention thus resolves all the problems related to the earlier embodiments. In particular, the battery charging circuit is a system of charging without a protection diode against reverse voltages and overvoltages.